Most acoustic background noise at or near the location of a telephone is to some extent introduced into the network along with the desired transmitted speech. Frequently, the noise affects speech intelligibility, or is at least distracting to the distant party. In addition, digital coding of the speech content of a signal that contains substantial incoming noise is prone to data errors because of the noise content.
Previous approaches to reducing outbound acoustic noise cancellation have included passive expander circuits such are used in the electret-type telephonic microphone. These, however, suppress only low level noise occurring during periods when speech is not present. Further, expander circuits suppress no noise whatever during speech. Passive noise-cancelling microphones are also used to reduce outbound noise. These have a tendency to attenuate and distort the speech signal when the microphone is not in close proximity to the user's mouth; and further are typically effective only in a frequency range up to about 1 KHz.
Active noise-cancellation circuitry to reduce outbound noise has been suggested which employs a noise-detecting reference microphone and adaptive cancellation circuitry to generate a continuous replica of the outbound noise signal that then is subtracted from the total outbound signal before it enters the network. Most such arrangements still are not sufficiently effective, however. They are susceptible to cancellation degradation because of a lack of coherence between the noise signal received by the reference microphone and the noise signal impinging on the transmit microphone. Their performance also varies depending on the directionality of the noise; and they also tend to attenuate or distort the speech.